SEDIMENT DYNAMICS OVER AN INTERTIDAL ZONE

9.1 INTRODUCTION

Researchers have studied Freiston Shore since the 1950s, with a variety of studies providing a background to the present investigation (Table 3.1). Data are available prior to the land reclamation of 1980, since the reclamation, and following MR (2002) of the original 1980 reclamation site. As such, an extensive data set is available to investigate the impact of anthropogenic changes, on the hydrodynamics and geomorphology of the intertidal zone. The aim of this Chapter is to describe the sediment dynamic and hydrodynamic conditions over the intertidal zone, and, wherever possible, to compare the present situation with those of the past to identify any changes that have occurred over the last 35 years. Understanding the natural processes, in particular the tidal currents, wave climate, sediment transport and sediment stability, over intertidal flats is necessary prior to the initiation of any coastal management (Gao and Collins, 1995). Once an understanding of the natural processes is established, any impacts from anthropogenic changes on the intertidal zone can be identified. These impacts need to be recognised to predict how different forms of coastal management will effect the surrounding natural environment, which is important for deciding on suitable forms of coastal management for the future. This Chapter utilises an increase in the accuracy of measurements, which newly available equipment has provided, to attempt to refine the past findings and thus improve the general understanding of the environment.

The wave conditions over the intertidal flats and inside the MR site were measured, to examine the effectiveness of the different sub-environments of the intertidal flats at

dissipating wave energy. Factors affecting suspended sediment concentrations in the waters overlying the intertidal zone were investigated, as it was found previously that storm conditions in the North Sea could increase the level of suspended sediment ten-fold over the intertidal flats at Freiston Shore (Evans and Collins, 1975). However, previous studies have not identified the effect of locally varying tidal currents and wave conditions on the

suspended sediment. At other intertidal flat locations, it has been shown that any simple empirical relationship between SSC and tidal height, related strongly to the tidal current, is generally obscured (French and Spencer, 1993). There are a number of possible reasons for this lack of correlation: the deposition and resuspension of sediment during the tidal cycle; sediment mobilisation by wind-waves; and any variations in the background concentration of sediment, from offshore areas (Evans and Collins, 1987; and Luternauer et al., 1995). Measurements of erosion/accretion have been used in the present investigation to identify seasonal changes in bed level and their spatial variability over the flats. The associated

compositions of the surficial sediments were analysed, then compared with the rates of erosion/accretion change, to identify any existing correlations. These data have been

combined with those from an earlier study, where no definite trends in bed level change were identified (Amos, 1974), to establish any patterns in the combined data and to see if there has been any change over time.

9.2 METHODS

Data collected by the ABRs and the sediment samplers, during 4 deployments, have been utilised (Section 4.3). Supplementary hydrodynamic data have been used: (a) from the wave buoy at the mouth of The Wash; and (b) from 3 wave/tide recorders deployed across the intertidal zone, at Butterwick Low (Section 4.8.5). Erosion/accretion data were used, together with surficial sediment samples obtained from the monthly sampling programme, which extended over an 18-month period (Section 4.4). Supplementary erosion/accretion measurements were collected over the saltmarsh (Section 4.8.3).

The high-frequency data collected from the ABRs were processed using the WAVELOG TURBIDITY 1.02 software (made available by Valeport Ltd. 2002). The processing involved:

(i) the calculation of the mean level and tidal slope for each burst;

(ii) the de-trending of the bursts, to remove the low-frequency tidal components; (iii) the application of a frequency-dependent correction to the pressure fluctuations, to

correct attenuation at the high-frequency end of wave spectrum; and

(iv) the calculation of the summary wave parameters and the frequency spectrum, from the corrected water surface record.

Synoptic wave conditions at the different measured sites were then compared, to calculate the extent of wave dissipation over the intertidal zone, in accordance with the procedure of Moeller et al. (1996).

9.3 RESULTS 9.3.1 Hydrodynamics

The sampled tidal heights were compared to those which occurred from 2002 to 2004 to examine their representation of wider-ranging conditions (Fig. 9.1). This comparison showed the tides sampled during this study represent the upper part of the tidal ranges, i.e.

Tidal Height (m O.D.) 1.0 - 1 .3 1.4 - 1 .7 1.8 - 2 .1 2.2 - 2 .5 2.6 - 2 .9 3.0 - 3 .3 3.4 - 3 .7 3.8 - 4 .1 4.2 - 4 .5 > 4.5 Fre quen cy 0 100 200 300 400 500 (a)

Tidal Height (m O.D.) 1.0 - 1 .3 1.4 - 1 .7 1.8 - 2 .1 2.2 - 2 .5 2.6 - 2 .9 3.0 - 3 .3 3.4 - 3 .7 3.8 - 4 .1 4.2 - 4 .5 > 4.5 Fre quen cy 0 2 4 6 8 10 12 (b)

Figure 9.1: (a) Frequency of tidal (predicted) height, for all tides from 2002 to 2004; and (b) frequency of the tides measured during Deployments 1 – 4 (for details, see text).

The hydrodynamic conditions measured over the upper and lower intertidal zones, throughout Deployments 1 and 4 are shown in Figures 9.2 to 9.5. The peak tidal current speeds were generally associated with the first phase of the flood and the last phase of the ebb; this pattern was more emphasised over the lower intertidal zone. The current speed was higher over the lower (with peak values ranging from 0.3 to 0.5 m s-1), compared to the upper intertidal zone (with peak values ranging from 0.2 to 0.3 m s-1). The upper intertidal zone often experienced a third peak, of similar magnitude to the first phase and last phase peaks, just before HW; this peak occurred simultaneously with the peak flow entering the MR site through the channel within Breach 1 (Fig. 9.6). The peak water depth at the upper intertidal site did not coincide with that of the channel within Breach 1; water continued to flow into the MR site until the water level over the adjacent saltmarsh dropped to a certain elevation. This was owing to the fact that the channels within the breaches were not of a sufficient size to allow the quantity of water necessary to fill the MR site to the same water level as present over the adjacent saltmarsh (Chapter 6). This meant that water continued flowing into the MR site after HW was experienced on the saltmarsh, resulting in different tidal curves inside the MR site and over the saltmarsh (Chapter 5).

The tidal current speeds during Deployment 1 increased as the tidal range increased,

throughout the deployment. The first 3 tides, when the tidal height was increasing from neap to spring tides, had lower current speeds than those of the later tides during the peak spring tidal heights. However, tidal current speed was not related directly to the tidal height; this can be seen clearly over the lower intertidal zone during Deployment 1 (Fig. 9.3), when the peak tidal current speeds were experienced during tides 0.5 m lower (on 07/09/2002 am & pm) than the peak tidal height (09/09/2002 am & 10/09/2002 am).

The SSC peaked during the first and last phases of the tidal cycle, with more defined and consistent peaks over the lower intertidal zone. However, throughout Deployments 1 – 3, the OBS became saturated during the peak values of some of the tides; as such, the peak

concentrations were not recorded. The gain setting was altered for Deployment 4, when peak values were recorded. The SSC decreased in a shoreward direction; the lower intertidal zone (site 3) had peak concentrations of 154 to 294 mg l-1, whilst the upper intertidal zone (site 1) had peaks of 108 to 255 mg l-1. The percentage of the spring tides at which the OBS became saturated, over Deployments 1 – 3, decreased shorewards; the OBS became saturated during 45 % of the tides measured over the upper intertidal zone, and during 73 % of the tides measured over the lower intertidal zone.

Wave conditions influenced the SSC, at both of the sites. Overall, the SSC appeared to vary more as a result of the prevailing wave conditions, than in response to tidal currents. The wave conditions varied over the deployments, with the significant wave height (Hs) ranging from 0.05 to 0.5 m; in comparison, the peak wave period (Tp) varied from 1.5 to 9 s. Hs was greater over the lower intertidal zone than the upper location; this was in response to wave energy being dissipated over the intertidal flats (discussed in more detail in Section 9.3.2).

The ratio between the duration of the flood and ebb phases of the tide, over the upper intertidal zone, was 0.8 : 1. Over the lower intertidal zone, at lower tidal ranges, the ratio of flood to ebb was similar (Fig. 9.7); during spring tides, with larger tidal ranges, the ratio was 0.64 : 1 (Fig. 9.8). The asymmetry of the tide varied temporally over the lower whilst it remained relatively constant over the upper intertidal zone.

Figure 9.2: The hydrodynamic and sediment dynamic conditions during Deployment 1, over the upper intertidal zone (site 1).

Figure 9.3: The hydrodynamic and sediment dynamic conditions during Deployment 1, over the lower intertidal zone (site 3).

Figure 9.4: The hydrodynamic and sediment dynamic conditions during Deployment 4, over the upper intertidal zone (site 1).